const {Readable} = require("stream");

class MyReadable extends Readable{
    constructor(){
        super();
        this.currentChar = 'a';
    }
    _read(){
        if(this.currentChar <= 'z'){
            this.push(this.currentChar);
            this.currentChar = String.fromCharCode(this.currentChar.charCodeAt(0)+1);
        }else{
            this.push(null);
        }
    }
}
const myReadable = new MyReadable();

myReadable.pipe(process.stdout);